A. Field of the Invention
The present invention related to a system and method for providing a time varying gain TDR to display abnormalities in a communication and telephone cable or the like by normalizing the levels of the reflected signals corresponding to a predetermined observation range. The communication cable can be any copper pair line that can be used for IDSL, ADSL, HDSL, SDSL, SHDSL, and VDSL communication usage, as well as all other DSL-type technologies (hereinafter, “xDSL” indicating all the various DSL technologies and line codings).
B. Description of the Prior Art
Internet access providers, cable communication companies, etc. are constantly working to fill demands for real-time internet access through the installation of fiber-optic and other hi-speed communications lines. However, in many areas of the country, such equipment and the services to support that equipment are either impractical to implement, prohibitively costly, or simply not scheduled to occur in the foreseeable future.
Telephone companies have tried to fill part of the demand by offering Digital Subscriber Line (xDSL) services that use the current infrastructure of copper pair lines to deliver hi-speed access to the Internet. Lines that work fine for standard telephone communication do not always work for different types of DSL operation.
The definition of copper pair lines includes any communication line made of copper or other similar material or composition known in the art. The proper conditions have to exist in order for a copper pair line to handle xDSL communications. The efficiency of a copper pair line for xDSL service is dependent on factors such as the length of the telephone line, the number of bridge taps on the line, material defects or shorts in the line, the wire gauge of the line, damage to the lines, proximity of sources of electromagnetic energy, etc.
Time Domain Reflectometers or TDRs are in common use in testing the abnormality of telephone and coaxial cables, such the TDR described in U.S. Pat. No. 5,461,318 to Borchert et al. (Oct. 24, 1995) which is a method for detecting impedance discontinuities in a two-conductor cable. However, using conventional TDR techniques, this process involves sending service technicians to the ends of a physical line then analyzing the received signal via a local switch. As one can imagine, this entire process is time consuming, labor intensive and costly.
TDRs detect fault anomalies such as opens, shorts, bridged-taps and wet sections. As these lines become longer the loss of the line is higher and it becomes increasingly harder to detect these anomalies. In most cases it takes considerable training and practice to discriminate between these various anomalies. Often even the most experienced telephone technician must drive to other locations, disconnecting sections to resolve their problems.
One key factor in differentiating between a short, a wet section and bridged-tap is the size of the return trace. “Trace” is a graphical representation of the line voltage verse time. However since the size of this return trace is related to the cable type and distance, or in other words the cable loss (with the return trace decreasing in amplitude as the distance increases), there is no existing method used to definitely resolve these common cable plant problems. Loss with cable plants of mixed cable types, makes detection for these faults even more complex. Detecting multiple faults at different lengths cannot be done on the same trace, as the user must manually set the gain for only a particular range of interest.
Another factor that masks the faults is a phenomenon called back-scatter decay. As cables become longer this returning signal becomes dominant over any detectable fault. Manual gain and offset controls are often required to see faults at longer distances. The user of a traditional TDR have to manually change the gain on the TDR trace in order to see the faults.
Therefore, there currently exists a need for a system and method to test the abnormalities of the copper pair lines that avoids the displaying problems and limitations associated with the current techniques. There also exists a need for a system and method to test the abnormalities of the copper pair lines that can aid in automatically showing abnormalities within a predetermined observation range so as to make the tracing and repair of copper pair lines for xDSL service more efficient.
Time Varying Gain (“TVG”), a predetermined gain versus time relationship, has been applied in side scanning sonar systems for mapping the topography of a under water seabed. Acoustic tone bursts (pings) are transmitted through the water column toward a target area and return from the target area are picked up by a receiver transducer and processed for display. Return signals may vary due to unknowns such as temperature, salinity and clarity of the water column. If bottom returns are involved, such as in side looking sonar systems, different bottom types such as mud, sand or rock will return different signals. For instance, U.S. Pat. No. 4,198,702 to Clifford (Apr. 15, 1980) describes a time varying gain amplifier for a side scanning sonar system having a predetermined gain versus time relationship. The gain is substantially proportional to the square of the elapsed time measured from the last sonar trace initiating trigger signal.
U.S. Pat. No. 5,392,257 to Gilmour (Feb. 21, 1995) further provides a sonar receiver with a normalizing processor circuit to modify/normalize the reflected signals to be displayed within the same range by adjusting gain levels. Normalizing processor circuit means is provided and is adapted to receive the signals reflected from unknowns in the water column and various bottom types, and the means is operable to generate an average error signal as a function of time. This error signal is applied to modify the output of the time varying gain circuit.
Adjusting gain levels is common in the art of data processing, and it has been applied to an optical time domain reflectometer (“OTDR”) in an optical measurement instrument. U.S. Pat. No. 4,893,006 to Wakai et al. (Jan. 9, 1990) applies such a level adjusting function to an OTDR which works in conjunction with an optical fiber. The optical time domain reflectometer tests a target optical fiber by sending an optical trace to the target optical fiber and detecting Fresnel reflection light and backscattered light returning from the fiber. A level changing means changes the level of the electric signal corresponding to a predetermined location of observation range so as to avoid saturation of the electric signal in the amplifier. As another example, U.S. Pat. No. 5,929,982 to Anderson (Jul. 27, 1999) applies such a level adjusting function (gain control) to an OTDR for optimizing the gain of an active avalanche photo-diode (“APD”). Any system noise is compared to a threshold value for establishing the optimum bias for optimum gain of the APD thereby to increase the dynamic range of the OTDR.
However, the application of TVG to a TDR for telephone lines of the present invention is unique, and the application simplifies the use of TDR in testing for abnormalities in telephone and coaxial cables and enables the display of multiple faults at various cable lengths. In addition, detecting multiple faults at different lengths can be done on the same trace automatically.